1. Field of the Invention
The present invention relates to a photodiode structure and the method for making the photodiode. In particular, the present invention relates to a photosensitive PIN photodiode structure and the method for manufacturing the photodiode.
2. Description of the Prior Art
The conventional copper cables are less and less likely capable to carry more and more signals to travel a longer and longer distance due to the physical limitation of electrical resistance and signal delays. Naturally, optical fibers meet the demand of carrying very large information to travel a very long distance so they replace the conventional copper cables to be the medium of long distance carrier of information because one single optical fiber allows multiple beams of light of different wavelength, each carrying different information to travel at the speed of light without mutual interference and without attenuating too much after traveling an extreme long distance.
Light of different wavelengths in the form of pulse signals constitutes the basic principle of transmission by optical fiber. However, such basic principle of transmission is not compatible with the basic principle of transmission by electron current in the current electronic devices to carry and to transmit signals. In order to form a transform medium between the optical fiber transmission and the electron current transmission, the photo-detector is deemed to be a convenient tool.
The photo-detector is an important photo-electrical transform unit. The photo-detector is capable of transforming the optical signals to electrical signals (into voltage or current), so it can transform the optical pulse signals in the optical fibers to become the electrical signals which can be carried, transmitted or used by ordinary electronic devices. Amongst them, the PIN (p-intrinsic-n photodiode) which has the advantages of easy to be manufactured, high reliability, low noise, compatible with low-voltage amplifier circuits and very wide bandwidth becomes one of the most widely used photo-detector.
The basic operational mechanism of the PIN photodiode is that when the incident light hits the p-n junction of the semiconductor, the electrons in the valence band of the semiconductor would absorb the energy of the photons in the incident light and jump over the forbidden band to arrive at the conduction band, which means, the incident photons create electrons, called photo-electrons, in the conduction band of the semiconductor if the photons have sufficient energy. Simultaneously, an electrical hole is left behind in the valence band and an electron-hole pair, or called photocarrier, is thus generated, which is also known as the photoelectric effect of the semiconductors. Afterwards, the photo-electron and the corresponding hole are quickly separated under the influence of an inner electric field and an outer negative bias to be respectively collected at the positive electrode and the negative electrode. Therefore, a photo-current appears in the outer circuit.
In order to enhance the operational performance of the PIN photodiode, the current technology integrates the Ge semiconductor material into the Si substrate to accomplish an optical communication of wide wavelength because Ge is deemed to have much higher carrier mobility than Si. The importance of integration of Ge semiconductor material into the Si substrate lies in the essential qualities of fast, effective and low noise. The photo-detectors made of Ge have the capabilities of effectively detecting the optical signals at the wavelength used by the optical communication. In addition, if the photo-detectors made of Ge are integrated with the conventional processes of Si type, it would be able to further lower the cost of the PIN photodiode.
There is a known PIN photodiode which integrates the Ge semiconductor material into the Si substrate. FIG. 1 illustrates the conventional PIN photodiode with Ge semiconductor material. The PIN photodiode 101 includes a Si substrate 110, an oxide layer 120, a P-doped Si 130, the intrinsic Ge 140, an N-doped Si 150, electrode regions such as the first electrode region 161 and the second electrode region 162. The P-doped Si 130, the intrinsic Ge 140 and the N-doped Si 150 together constitute the core element of the PIN photodiode. Because in the above-mentioned structure of the PIN photodiode 101, the first electrode region 161 in the electrode regions is disposed above the N-doped Si 150, such arrangement will decrease the frontal area to receive light and the quantum yield is thus lower due to the incident light partially absorbed by passing through the p-doped Si 130. Moreover, the manufacturing process of the PIN photodiode 101 is not fully compatible with that of the conventional MOS. Accordingly, it is needed to provide a novel PIN photodiode structure and the method for making the PIN photodiode to more effectively integrate the manufacturing process of the novel PIN photodiode structure with the traditionally fully-developed MOS manufacturing process to lower the manufacturing cost.